1. Field of the Invention
This invention relates to fabrication methods used for semiconductor devices and more specifically to an improved process to create barrier films of the type used to separate aluminum-based metallizations from silicon regions contacted thereby.
2. Description of the Related Art
The semiconductor industry continually strives to increase device performance while maintaining or decreasing the cost of manufacturing advanced silicon chips. These objectives have been realized, in part, by the ability of the semiconductor industry to achieve silicon device miniaturization. The ability to produce silicon devices with sub-micron critical features has resulted in significant device performance improvements, for example by reducing the resistance of metal interconnect wiring, as well as reducing capacitance with underlying silicon device regions. Miniaturization has also contributed to cost reductions. The ability to reduce critical device dimensions has led to a significant reduction in chip size, allowing more chips to be produced from a specific silicon wafer size, thereby reducing the cost per chip.
The attainment of miniaturization relates to advances in semiconductor fabrication. Advances in photolithography including advanced exposure cameras as well as more sensitive photoresist materials have enabled sub-micron images in photoresist to be routinely achieved. Similar advances in selective dry etching and particularly anisotropic reactive ion etching (RIE) have allowed sub-micron images in photoresist to be successfully transferred to the underlying device layers during the fabrication of semiconductor chips. Other fabrication disciplines such as ion implantation and low pressure chemical vapor deposition (LPCVD) have also been major contributing factors in the miniaturization of the device.
However, problems arise with continued miniaturization. As the metal filled contact holes used to connect overlying interconnect wiring lines to underlying silicon regions become smaller, undesirable phenomena that are not observed in larger contact hole structures can occur. For example, it can be difficult to form an effective barrier layer at the bottom of a contact hole which has a large aspect ratio, such as may arise for deep holes having openings of about 0.5 .mu.m across or smaller. An effective barrier layer is most preferably formed between a doped silicon region and the metal line that contacts the doped silicon region. Effective barrier layers are particularly important when the metal line which contacts the doped silicon region includes a comparatively high aluminum concentration. For such connections, an effective barrier layer limits the extent of aluminum penetration into underlying silicon device regions. Such penetration, known as "spiking," can result in junction degradation, particularly when the aluminum "spike" penetrates through a source/drain region, allowing signals on the source/drain region to couple more directly to the substrate.
The barrier layer, in most cases a titanium nitride (TiN) layer, is usually formed on a titanium disilicide underlying layer, which enhances the ohmic nature of the contact between the metal interconnect line and the doped silicon regions. Conventionally, titanium disilicide is produced by directly depositing of a titanium layer onto an exposed silicon region, followed by one or more heat cycles to convert the titanium to titanium disilicide and to modify the crystalline structure of the materials formed in the initial part of the reaction. In some cases, the titanium silicide is formed in a nitrogen or ammonia ambient, thus producing a titanium disilicide layer at the interface between the deposited titanium layer and the doped silicon region at the bottom of the contact hole. A TiN barrier layer is created in the same processing steps used to form the underlying titanium disilicide. This process, sometimes referred to as rapid thermal nitridation (RTN), forms a limited thickness of TiN. TiN forms as a layer over the deposited titanium so that subsequent reaction of the nitrogen ions with the unreacted titanium requires the diffusion of nitrogen through the TiN layer to the unreacted titanium. The TiN layer acts as a barrier to the diffusion of nitrogen or ammonia so that, after an initial thickness of TiN is formed, additional TiN is formed slowly. As a practical matter, then, only a given thickness of titanium nitride layer can be formed in a manner that is readily compatible with the demands of mass production.
A different method that can be used to create TiN barrier layers uses reactive sputtering. A titanium disilicide ohmic contact layer is formed by deposition of titanium followed by heat treatments which convert titanium to titanium disilicide. The TiN barrier layer is then deposited by sputtering titanium in a nitrogen containing ambient under conditions that allow the titanium to react with the titanium prior to or during deposition to form titanium nitride. This method undesirably requires two separate sputtering chambers, since the reactive sputtering process can cause significant amounts of TiN to deposit on the titanium target. If a titanium target contaminated with titanium nitride is used to deposit what should be pure titanium, titanium nitride may instead deposit and the deposited material may not be suitable for forming a good ohmic contact with the silicon region. In addition, the amount of material that can practically be sputtered into the contact hole by this method is limited by the high aspect ratio of the contact hole.